In recent years, sealed batteries have widely been used. Examples of such sealed batteries include aqueous electrolyte batteries typified by high-capacity alkaline storage batteries and nonaqueous electrolyte batteries typified by lithium-ion batteries which are increasingly used as power sources for driving portable electronic devices or other devices. Moreover, with increase of functions of the electronic devices and communication devices in recent years, sealed batteries with higher capacity have been in demand. As the capacity of the sealed batteries increases, measures for safety are to be emphasized. In particular, internal short-circuits, or the like in sealed batteries may cause a rapid temperature rise, which may lead to thermal runaway. Thus, it is strongly demanded to improve the safety. In particular, large-size, high-power sealed batteries require the technique of, for example, reducing the thermal runaway in order to improve the safety.
These sealed batteries have a sealed structure in which an electrode group formed by winding or stacking a positive electrode plate and a negative electrode plate with a separator interposed between the positive electrode plate and the negative electrode plate is housed in a battery case together with an electrolyte, and in which an opening of the battery case is sealed with a sealing plate with a gasket sandwiched between the opening and the sealing plate. In this structure, a lead extending from one of the electrode plates (e.g., the positive electrode plate) in the electrode group is connected to the sealing plate serving as an external terminal at one side, whereas a lead extending from the other electrode plate (e.g., the negative electrode plate) in the electrode group is connected to an inner surface of the battery case serving as an external terminal at the other side. To connect the lead to the sealing plate or to the inner surface of the battery case, resistance welding is widely employed.
The opening of the battery case is sealed by resistance-welding the lead extending from the electrode group to the sealing plate, with the electrode group being housed in the battery case, and then bending the lead to be housed in the battery case to seal the opening of the battery case with the sealing plate. In this process, while the lead extending from the electrode group is resistance-welded to the sealing plate, substances (mainly metal particles removed from a welded portion of the lead) can be sputtered. If these sputtered substances enter the electrode group in the battery case, the separator might be damaged, resulting in an internal short-circuit. In another case where sputtered substances adhere to the gasket joined to the periphery of the sealing plate, when the opening of the battery case is sealed with the sealing plate by crimping with a gasket sandwiched between the opening and the sealing plate, a portion of the gasket narrowed by crimping might be sheared by the sputtered substances. Consequently, the battery case and the sealing plate come into contact with each other while sandwiching the sputtered substances therebetween, resulting in a short circuit.
To prevent such a short circuit caused by, for example, contamination by sputtered substances, the opening of a battery case may be covered with a thin plate or the like during production so as to prevent sputtered substances from entering the battery case, for example, during resistance welding of the lead extending from the electrode group to the sealing plate. However, the opening cannot be completely covered, and thus, such covering is insufficient for preventing contamination by sputtered substances.
On the other hand, joining by ultrasonic welding, instead of resistance welding, does not cause melting as caused by the resistance welding, and thus contamination by sputtered substances can be prevented in principle. However, joining by ultrasonic welding exhibits a lower joint strength than that obtained by the resistance welding. In addition, if the sealing plate has a safety mechanism for explosion protection, ultrasonic vibration might affect the function of the safety mechanism, or might cause peeling off of an active material from the electrode plate. Thus, joining by the ultrasonic welding is not preferable in reliability.
Since aluminum is generally used as a material for current collectors of positive electrode plates of lithium ion secondary batteries, the leads extending from the positive electrode plates also use aluminum. In addition, to reduce the weight of batteries, the battery cases and the sealing plates have begun to use aluminum. In this case, welding between the lead and the sealing plate means connection between aluminum components. In general, aluminum has a higher electric conductivity and a higher thermal conductivity than those of steel. Accordingly, a large current needs to flow for a short period in resistance welding of aluminum components, resulting in that a welding rod used in the resistance welding wears worse in aluminum welding than in steel welding, and it is difficult to maintain stable welding for a long period.
To prevent this problem, laser welding using a pulse oscillation YAG laser which is capable of locally concentrating energy is employed for welding between the lead and the sealing plate. Since a laser beam can be narrowed in the laser welding, the melted area can be smaller in the laser welding than in the resistance welding. Accordingly, the amount of sputtered substances can be reduced.
For example, Patent Documents 1, 2 disclose, as illustrated in FIGS. 7, 8, a method in which a lead 42 extending from an electrode group 41 is laser-welded to a sealing plate 40 by a pulse oscillation YAG laser to join the lead 42 to the sealing plate 40 at two or more welded portions 43.